Preserving sea slugs

There are many reasons why sea slugs, especially those without shells, are difficult to preserve in a way that makes them useful for future research. In most mollusc groups, the shape and colour of the shell is a very useful, and often all that is needed to identify a species. In slugs however, it is necessary to know the shape and colour of the living animal, two characters often lost on preservation. Slugs are without an internal or external skeleton. If they are not narcotised (put to sleep) before being killed and preserved, they tend to retract into a shapeless ball. When dissected, the internal organs of such animals are usually so misplaced and jammed together, that an understanding of their anatomy is very difficult. Here are some basic instructions on how to preserve specimens so that they are useful for future research..

Preserving and fixing specimens.
The first step is to narcotise the animals so they don't contract into an unrecognisable ball when placed in preservative. Two commonly used techniques are menthol crystals and magnesium salts.
Menthol crystals Put slugs in a shallow dish and float crystals on the surface of the sea water. It may take some hours for this method to work.
Magnesium salts Make a saturated solution of Magnesium chloride or Magnesium sulfate, and gradually add to seawater. This is a much quicker method and if you add carefully can give a good result. [Magnesium sulfate = Epsom Salts]

I'll just mention the two common methods for general anatomy. For histology and DNA work more specialised techniques are required.
Formalin. I fix my material in 10% formalin overnight then transfer to 5% formalin for longterm storage. Formalin fumes can be unpleasant and it can sting if you have cuts on your hands. During dissecting I transfer the animal to water so that I don't breathe in the formalin fumes.
Alcohol. Many workers store their material in 70% alcohol (usually ethanol). Two disadavantages I find, are that it hardens reproductive structures so they are difficult to untangle, and if the specimen is dissected in water, mucus glands when broken or cut begin to rapidly swell and can completely obscure the dissection. For some reason the hardening does not seem to happen with land snail specimens.

I can't say whether alcohol or formalin is best, both work quite well.

The most important point is to photograph specimens alive, and if possible draw them and make colour notes.

Authorship details
Rudman, W.B., 2000 (July 17) Preserving sea slugs. [In] Sea Slug Forum. Australian Museum, Sydney. Available from

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